1. Field of the Invention
The present invention relates to a hands-free telephone set providing a microphone and a loudspeaker for making telephone communication in hands-free. In particular, the present invention relates to a hands-free telephone set including an acoustic echo suppression system and a side-tone echo suppression system.
2. Description of the Related Art
Lately, a hands-free telephone set, which will be simply called a "hands-free telephone" hereinafter, comes into use because of convenience. By virtue of the hands-free telephone, telephone communication can be performed in hands-free, using a microphone, a loudspeaker and single chip digital signal processing integrated circuits associated with them.
In a telephone set, generally, an echo phenomenon occurs, and in an ordinary hand-set telephone set, an echo phenomenon occurs due to a side-tone echo produced by impedance mismatching at a well known two-wire/four-wire converter in the telephone set. However, in the hands-free telephone, not only the side-tone echo but also an acoustic echo due to sound coupling between the loudspeaker and the microphone is produced. Therefore, in the hands-free telephone, there are two echo suppression systems, an acoustic echo suppression system including the loudspeaker and the microphone and a side-tone echo suppression system including the two-wire/four-wire converter.
Since the microphone and the loudspeaker are apart from the mouth and ears of telephone talker, which will be simply called "talker" hereinafter, respectively, the hands-free telephone is susceptible to influences from surrounding noise and reverberation, in comparison with the ordinary hand-set telephone.
Further, since the microphone and the loudspeaker are arranged closely on the hands-free telephone, the acoustic echo caused by acoustic coupling between the microphone and the loudspeaker occurs easily. Still further, since the microphone is apart from the mouth of talker, voice sound coming from talker into the microphone becomes small, which causes the microphone to produce a small output signal. As a result, gains must be increased for amplifying the small signal, which causes to produce easily well known howling in the hands-free telephone.
Great efforts have been exerted for solving the above problems. FIG. 1 shows a block diagram of a most modern hands-free telephone 1' of the prior art, developed in consideration of the above problems.
In FIG. 1, a microphone (MIC) 10 receives voice of talker and produces an analog signal which will be called a "transmitting analog signal" hereinafter. The transmitting analog signal is sent to an analog digital converter (A/D) 21 by which the transmitting analog signal is converted to a digital signal which will be called a "transmitting digital signal" hereinafter. The transmitting digital signal is sent to an acoustic echo canceler (AEC) 60, which is a key device of the acoustic echo suppression system, by which an acoustic echo caused by acoustic coupling between MIC 10 and a loudspeaker 20 is suppressed. The transmitting digital signal having passed through AEC 60 is sent to a voice-switched variable attenuator (V-SW ATT) 90 whose function will be explained later. The transmitting digital signal having passed through V-SW ATT 90 is sent to a digital analog converter (D/A) 31 and partly to an sidetone echo canceler (SEC) 70 which is a key device of the side-tone echo suppression system and will be also explained later. In D/A 31, the transmitting digital signal is converted back to an analog signal called a transmitting analog output signal hereinafter. Then, the transmitting analog output signal is sent to a hybrid circuit (H) 50 which is a well known two-wire/four-wire converter used in a conventional telephone set. That is, the four-wire transmitting analog output signal is converted to a two-wire transmitting analog signal at H 50. The two-wire transmitting analog signal is sent to a local switch, not depicted in FIG. 1, through a two-wire telephone line 51.
When the hands-free telephone 1' receives a two-wire received analog signal from the local switch through the two-wire telephone line 51, the two-wire received analog signal is converted to a four-wire received analog signal at H 50. The four-wire received analog signal is converted to a received digital signal at an analog digital converter (A/D) 22. The received digital signal output from A/D 22 is sent to SEC 70 by which a side-tone echo caused by the leakage signal produced due to the impedance mismatching at H 50 is suppressed in cooperation with the transmitting digital signal sent to SEC 70 partly through V-SW ATT 90. The received digital signal having passed through SEC 70 is sent to V-SW ATT 90. The received digital signal having passed through V-SW ATT 90 is sent to a received signal amplifier (REC-SIG AMP) 81 at which the received digital signal is amplified to a manually controlled level, producing an amplified received digital signal. The amplified received digital signal is sent to a digital analog converter (D/A) 32 where the amplified received digital signal is converted to a received analog signal. The received analog signal output from D/A 32 is sent to a loudspeaker (SPK) 40 so that received telephone voice is loudly spoken from SPK 40. Hereupon, a part of the received analog signal output from REC-SIG AMP 81 is fed to AEC 60 for suppressing the acoustic echo.
In the above, the echo suppression performed by AEC 60 (SEC 70) is accomplished by synthesizing an echo replica and subtracting the echo replica from the output signal of MIC 10 (H 50). Wherein, the echo replica is a signal simulating the echo, produced in accordance with an estimation process performed through MIC 10 and SPK 20 in case of AEC 60 and through H 50 in case of SEC 70. By virtue of applying AEC 60 (SEC 70) to the hands-free telephone 1', the echo suppression can be performed well. However, it takes a processing time to perform the synthesization and the subtraction of the echo replica, and during the processing time, the hands-free telephone 1' becomes unstable in operation, so that the hands-free telephone 1' happens to fall into oscillation such as howling. The V-SW ATT 90 is an automatic attenuator mainly for preventing such oscillation from occurring in the hands-free telephone 1'. The V-SW ATT 90 compares amplitude of the transmitting digital signal and the received digital signal applied to V-SW ATT 90 for determining which digital signal is smaller and reduces amplitude of the smaller digital signal so as to maintain the hands-free telephone 1' in a stable state during the processing time.
A paper on the hands-free telephone 1', especially about AEC 60, SEC 70 and V-SW ATT 90, has been read in Abstracts of Meeting on Acoustic Engineering, held by Acoustic Society of Japan in March, 1990, titled "A study on loudspeaker telephone using small echo cancellets", by Hiroyuki Masuda, Kensaku Fujii and Juro Ohga who are inventors of the present invention. The same subject as the above is presented at 119th Meeting held by Acoustic Society of America, titled "Hands-free telephone using compact echo cancelers and voice-switched attenuators", by Juro Ohga, Hiroyuki Masuda, Kensaku Fujii and Yoshiro Sasaki, and the abstract of the presentation is published in J. Acoustic Soc. Am. Suppl. 1, Vol. 87, Spring 1990. Further, a paper theoretically explaining the acoustic echo canceler has been read in "Special Section on Acoustic System Modeling and Signal Processing", of IEICE TRANS. FUNDAMENTALS. VOL. E75-A, NO. 11 NOVEMBER 1992, with a title "A Fast Adaptive Algorithm Suitable for Acoustic Echo Cancellet", by Kensaku Fujii and Juro Ohga.
The AEC 60 is the key device of the acoustic echo suppression system. However, MIC 10 is another important device of the acoustic echo suppression system. Because, loosening the acoustic coupling between MIC 10 and SPK 40 is very important for suppressing the acoustic echo. However, since SPK 40 and MIC 10 are arranged closely, SPK 40 and MIC 10 easily form the acoustic echo coupling. Therefore, in order to prevent the acoustic echo coupling from occurring between SPK 40 and MIC 10, a bidirectional microphone system shown in FIG. 2(a) is applied to MIC 10.
In FIG. 2(a), the bidirectional microphone system of MIC 10 consists of a pair of omnidirectional microphones (OMN MICs) 10A and 10B, a pair of microphone amplifiers (MIC AMPs) 10a and 10b connected with omnidirectional microphones 10A and 10b respectively and an operational amplifier (OPE AMP) 10C connected with MIC AMPs 10a and 10b at a minus terminal and a plus terminal of OPE AMP 10C respectively. The bidirectional microphone system is for making MIC 10 receive sound directly from SPK 40 substantially as little as possible. Since the outputs from OMN MICs 10A and 10B are sent to OPE AMP 10C, when the outputs from OMN MICs 10A and 10B are equal to each other, no output signal (transmitting analog signal) is produced from OPE AMP 10C theoretically. Therefore, when OMN MICs 10A and 10B are arranged on the hands-free telephone 1' so that distances directly from SPK 40 to OMN MICs 10A and 10B are equal to each other, the output from OPE AMP 10C can be reduced. In this case, if talker speaks toward MIC 10 in a direction perpendicular to a line connecting OMN MICs 10A and 10B, the transmitting analog signal due to voice of talker can be produced from OPE AMP 10C in maximum. Because, the difference between talker's voice arrived at OMN MICs 10A and 10B becomes maximum, resulting in producing a maximum output from OPE AMP 10C. A bidirectional characteristic of MIC 10 (the bidirectional microphone system) is shown in FIG. 2(b). In FIG. 2(b), the bidirectional characteristic is represented by angular co-ordinates. The OMN MICs 10A and 10B are positioned on a line including 0.degree. and 180.degree. axes and origin "0" of the angular coordinates is placed at a middle point between OMN MICs 10A and 10B. The bidirectional characteristic shows in FIG. 2(b) that the output of MIC 10 becomes maximum at angle 0.degree. and 180.degree. and minimum at angle 90.degree. and 270.degree.. It can be realized from the bidirectional characteristic that SPK 40 should be placed in a plane including 90.degree.-270.degree. axis and perpendicular to 0.degree.-180.degree. axis for minimizing the acoustic echo and talker should speak toward point "0" along 0.degree.-180.degree. axis for obtaining the maximum output from MIC 10. In FIG. 2(b), a zone including 90.degree. and 270.degree. axes will be called a dead zone hereinafter.